Recently an effect of sequential electron transfer on electon spin polarized transient EPR spectra at high magnetic field from a natural photosynthetic system of purple bacteria has been investigated theoretically (J. Tang, S. Bondeson, M.C. Thurnauer, Chem. Phys. Lett 253:293-298, 1996). It has been shown that EPR at 95 GHz and higher frequencies can discriminate between different models of spin polarization. In order to check these theoretical predictions, we are designing a new W-band resonator suitable for in situ illunimation. We have already succeeded in building a ~50% transparent resonator consisting of a helix of silver wire on a plastic support. Modification of the cryogenic probehead to accommodate a light guide is under way.